1. Field of the Invention
The present invention relates generally to digital valves and more specifically to binary digital valves having interchangeable valve sizes and air powered cylinders.
2. Description of the Prior Art
Valves can be divided into two types: analog and digital. Analog valves are by far the most popular and widely used type of valve. Regulation of flow is accomplished by partially closing or opening a valve until the desired flow is achieved. This can be done manually or automatically. Manual control consists of simply turning the valve stem, as with a garden hose nozzle, until the proper volume flow is achieved. Analog valves can be controlled automatically by means of electric, pneumatic, or hydraulic actuators with enough power to position the valve against the fluid head and position sensors which detect the valve position.
Analog valves have several disadvantages. Mechanical clearances and friction in the valve control linkage can introduce hysteresis and uncertaintly in the valve setting. The valve linkage can be sensitive to the fluid forces acting on the valve stem, which can result in disproportionate changes in the fluid flow for changes in the fluid head. Also, the valve response time for analog valves is relatively slow. Changes in the valve setting normally require at least one second, and this applies whether large flow adjustments or small adjustments are desired.
A digital valve consists of a series of valves in parallel with the flow, each valve having a flow coefficient which is a binary multiple of that of the other valves. For example, a digital valve could consist of a group of six solenoid operated valves. The first valve could have a flow coefficient of 1.0, the second valve could have a flow coefficient of 2.0, the third valve could have a flow coefficient of 4.0, the fourth valve would have a flow coefficient of 8.0, the fifth valve would have a flow coefficient of 16.0, and the sixth valve would have a flow coefficient of 32.0. Each of these valves would be operated independently by an electrical signal.
By selecting the proper valve or combination of valves to be opened in the above example, any integral value of flow coefficient between 1.0 and 64.0 can be obtained. An appropriate control system, such as a computer or selector switch, would select a value for the valve opening. If this value is an analog word it would be converted into a digital word in an analog-to-digital converter. Each digit of this word would be directed to its corresponding valve and would direct that particular valve to open or close. Thus, the flow coefficient could be set in digital increments for any desired flow.
Prior art patents which deal with digital valves include: U.S. Pat. No. 4,l70,245, by Frank P. Haley; U.S. Pat. No. 4,356,840, by Harry Friedland, et al; U.S. Pat. No. 4,244,396, by Harry Friedland, et al; U.S. Pat. No. 3,942,553, by Robert A. Gallatin; U.S. Pat. No. 3,726,296, by Harry Friedland, et al; U.S. Pat. No. 3,886,971, by Jorgen Lundsgaard, et al; U.S. Pat. No. 4,577,658, by Michael Bosteels, et al; U.S. Pat. No. 4,256,100, by Donald Levy, et al; and U.S. Pat. No. 4,313,465, by Heinz Holzem, et al. Another type of digital valve is disclosed by U.S. Patent Office Defensive Publication No. 692,145, published Dec. 24, 1968, by James Cobb, et al.
One problem with current valves involves operation with high pressure fluids. At higher pressures, more force is needed to lift the valves off of their valve seat. Solenoid powered valves are limited in the amount of force they can exert and are impractical in high pressure fluid applications. Air powered valves are one solution. However, a common source of air pressure, such as shop air sources, usually are no greater than one hundred psi. If a six hundred psi fluid system is being used, then an air diaphragm with an area at least six times the area of the valve would be needed to lift the valve, if air pressure of only one hundred psi is available. In a digital valve system, consisting of several binary valves, the large diaphragms required for each valve would make the entire valve unit very bulky.
Another way to operate a digital valve at high pressures involves using a pilot orifice. In this application, the main valve has a small pilot orifice which can be opened with a solenoid. Once a pilot orifice is opened, a differential pressure exists across the main valve in a direction to open the valve. To close the main valve, the pilot valve is closed, and pressure builds up on top of the main valve and the main valve closes.
Pilot valves have a number of problems. Most depend on positive fluid pressure to work. If there is a negative pressure differential then the main valve will open and remain open even through the control indicates "closed". Another problem involves the pilot orifice. This small passageway must be kept clean, but in certain applications, such as the chemical industry, this pilot orifice can easily become clogged with impurities. This would render the valve inoperable, and would necessitate frequent cleaning of the entire valve assembly.
Another problem with the digital valves of the prior art involves their inflexibility in application. The prior art valve units are built with a certain number of valve orifices, each of which has a certain area. If a different range of flow rates is needed, an entirely different valve with a different number of valve orifices of different sizes may be required.
What is needed is a compact size digital valve unit which can be used in high pressure systems. This valve unit should be constructed such that different numbers of valves and different sizes of valve orifices can be interchanged such that the valve unit can accomodate a variety of applications.